Steam generating unit



Sept. 21, 1965 E. C. MILLER STEAM GENERATING UNIT Filed OCT.. 22, 1964 5 Sheets-Sheet l INVENTOR Earle C. ZiZZer H Tney Sept. 21, 1965 E. c. MILLER STEAM GENERATING UNIT 3 Sheets-Sheet 2 Filed Oct. 22, .1964

va -lz INVENTOR Earle C. YZI'ZZer H o'rney Sept. 21, 1965 Filed OCT.. 22, 1964 E. C. MILLER STEAM GENERATING UNIT 5 Sheets-Sheet 3 cLosfo fion-N cLoseo OPEN LOW LOA D k OPEN CLOSED OPEN nasen HIGH LOAD Inj. 6

INVENTOR. Earle C. YZI' ZZer H ey United States Patent O '3,207,134 'STEAM GENERATING UNIT Earle C. Miller, Worcester, Mass., assignor to Riley Stoker Corporation, Worcester, Mass., a corporation of Massachusetts 'Filed Oct. 22, 1964, Ser. No. 405,817 8 Claims. (Cl. 1212-479) This is a continuation-in-part of my co-pending patent application Serial Number 196,783, filed May 22, 1962, now abandoned.

This invention relates to a steam generating unit and, more particularly, to apparatus arranged to produce `steam by burning solid fuel on a traveling grate.

In the generation of steam, one of the major problems is maintaining the temperature of superheated steam passing to the turbine at a constant, predetermined value. The difficulty arises because of the tendency of a conventional steam generating unit to produce steam at a lower temperature at low loads. It has been possible in late years to construct steam generating units making use of the combustion of solid fuels on a traveling grate with very large capacities. When a unit of this type is built with a high capacity, the turbine is necessarily more sophisticated and the regulation of the temperature of superheated steam become more important. Nevertheless, the methods for controlling superheat in such units has been limited to methods which are either expensive or ineffective. Most of the means which are used in other types of steam generating units for controlling superheat without lowering the efciency `of the unit are not applicable to the spreader Stoker traveling grate type of furnace because of their very nature. These and other difficulties experienced with the prior art devices have been obviated in a novel manner by the present invention.

It is, therefore, an outstanding object of the invention to provide a steam generating unit of the type which burns a solid fuel on a traveling grate and in which superheat is controlled by means of an inexpensive apparatus without loss of efficiency of the unit.

Another object of this invention is the provision of a steam generating unit of the spreader-Stoker travelinggrate type wherein the temperature of superheat is controlled by making use predominantly of conventional equipment found in this type of unit.

A further object of the present invention is the provision of a steam generating unit in which solid fuel is burned on a horizontal grate having apparatus for the control of superheat, which apparatus is simple and rugged in construction and is capable of a long life of useful service.

With these and other objects in view, as will be apparent to those `skilled in the art, the invention resides in the combination of parts set forth in the specification and covered by the claims appended hereto.

The character of the invention, however, may be best understood by reference to one of its structural forms, as illustrated by the accompanying drawings, in which:

FIG. 1 is a vertical longitudinal sectional view of a steam generating unit embodying the principles of the present invention;

FIG. 2 is a sectional view of the apparatus taken on the line II-II of FIG. 1;

FIG. 3 is a sectional view of the apparatus taken on the line III-III of FIG. 2;

FIG. 4 is a sectional View of the apparatus taken on the line IV-IV of FIG. 2;

FIG. 5 is a schematic View of the apparatus showing the mode of operation at low load; and

3,207,134 Patented Sept. 21, 1965 FIG. 6 is a schematic view of the apparatus showing the operation at high load.

Referring first to FIG. l, wherein are best shown the general features of the invention, the steam generating unit, indicated generally by the reference numeral 10, is indicated as comprising a furnace 11 and a boiler 12 mounted on a structural support 13. The furnace is provided wih a front wall 14, a rear wall 15, side walls 16, a roof 17, and a traveling grate 18 defining a combustion chamber 20. The Stoker 18 is similar in construction to that shown in the patent of Rivers No. 2,804,834 and is supplied with solid fuel by a series of spreader stokers 19 which are of the type shown and described in the patent of Miller No. 2,538,944. Also mounted on the front wall 14 is a series of pneumatic spreader stokers 21 supplied with refuse fuel or the like from a rotary distributor 22, this apparatus being similar to that described in the patent of Miller No. 2,925,055.

The boiler 12 is provided with a steam-and-water drum 23 and a lower drum 24 joined by downcomer tubes 25. From the lower drum 24 extends large downcomer tubes 26 which connect to headers 27 underlying the side wall 16, to a header 28 underlying the rear wall 15, and to a header 29 located in the front wall 14 just above the spreader stokers 19. Water-wall tubes leave the header 29 and extend upwardly along the inner surface of the front wall 14 and along the under-surface of the roof 17 to the steam-and-water drum 23, these tubes being spread in the vicinity of the pneumatic spreader stokers 21 to provide openings for the introduction of fuel. Similar water-wall tubes leave the header 28, form a small arch over the discharge end of the grate 18, and extend up the forwardly-facing surface of the rear wall 15. Some of these rear water-wall tubes extend forwardly in the upper part of the furnace to provide a screen 31 supporting a forwardly-extending bafe 32, while the remainder of the tubes extend directly upwardly in separate fashion to define a gas off-take 30. All tubes on the rear wall eventually terminate in the steamand-water drum 23. In a similar manner, water-wall tubes leave the side wall headers 27, extend upwardly along the inner surface of the side wall 16, and eventually discharge into the steam-and-water drum 23. A baille 33 extends vertically downwardly from the lower part of the drum 23 and defines a forward back pass 34 and a rear back pass 35. The downcomer tubes lie in the rear back pass 35. The back pass 35 leads to an economizer 31 which is connected to a dust collector 37 which discharges gas to a tubular air heater 38 whose outlet is connected to an induced draft fan 39 which is, in turn, connected to a breaching 41 leading to a stack (not shown). Underlying the dust collector 37 are hoppers 42; under the back passes 34 and 35 is mounted a hopper 43. These hoppers are all connected to the rear wall 15 by a flyash reinjection system 44. The unit is provided with a forced-draft fan 45 connected to the air inlet of the air heater 38. The air outlet of the air heater is connected by a duct 46 through a venturi 47 to an air plenum chamber 48 underlying the grate 18. An ash discharge pit 49 underlies the rear end of the grate 18 adjacent the rear wall 15.

The steam space of the steam-and-water drum 23 is connected to a convection superheater 51 located in the forward back pass 34, this superheater being connected to another convection superheater 52 located in a horizontal pass between the baflle 32 and the roof 17 just forwardly of the gas off-take 30. The convection superheater 52 is connected by a pipe 53 to a radiant superheater 54 located adjacent the front wall 14 and terminating at its lower end in a superheated steam header leading to the turbine (not shown). In the pipe 53 leading to the radiant superheater 54 is located a desuperheater 56 which is operated by signals in a line 57 connected to the output side of a main control 58. The main control is of the conventional type which receives input signals and converts them to output signals in a predetermined manner; the details of the control do not constitute part of the present invention. A temperature-indicating device 59 is located in the superheated steam header 55 and is connected by line 61 to the input side of the main control 58. Located in the venturi 47 are pressure-indicating devices 62 and 63 located at substantially different diameters of the venturi and connected by lines 64 and 65, respectively, to the input side of the main control 58; the difference in pressure between the two devices 62 and 63 is an indication of air flow through the duct 46 and, therefore, of load on the unit.

The upper run of the grate 18 extends over four air zone chambers 66, 67, 68, and 69, the chambers having air control valves connected by suitable means to linear actuators such as hydraulic cylinders 71, 72, 73, and 74, respectively. These cylinders are suitably connected by hydraulic lines to the output side of the main control 58.

FIGS. 2, 3, and 4 show the details of the typical air zone chamber 67. To begin with, the top of the chamber is defined by the bottom surface of the grate 18 which is supported on vertical I-beams '75 and 76. Extending along the bottom ends of the channels 75 and 76 is a horizontal bottom plate 60 which, in the preferred embodiment, is composed of a number of separate elements. Extending through the bottom plate of the chamber 67 are a series of openings 70, each opening having a damper 77 overlying it. The dampers in the chamber 67 are joined by an actuating rod 78 which, in turn, is connected to the piston rod of the hydraulic cylinder 72. As is evident in FIG. 2, each damper 77 appears in plan view as a rectangle which is elongated in the direction of movement of the chain grate from the front to the back of the furnace. In cross section, as is evident in FIG. 4, it is tent-shaped to provide very little surface for the accumulation of yash and cinders which filter down into the air Zone chamber from the traveling grate. Such flyash falls onto the upper surface of the bottom plate 60 between the openings '70 and is scraped through the openings by the movement of the dampers; this flyash falls into the bottom of the air plenum chamber 48 which is shaped as a hopper. lt should be noted that the bottom plate 60 is provided with upstanding guide channels 79 and 81 which extend entirely across the furnace from side wall to side wall and which retain the dampers 77 in their proper positions. Furthermore, reinforcing channels 82 are bolted on the under surface of the bottom plate along the edges of the openings 70 to maintain the bottom plate in a rigid condition so that a suitable seal is formed between the bottom edges of each damper 77 and the upper surface of the bottom plate 60 in the vicinity of its particular opening 70.

The operation of the invention will now be readily understood in view of the above description. ln the operation of the invention, the induced draft fan 39 draws gas from the boiler; the forced draft fan 45 supplies air to the unit for combustion. The spreader stokers 19 and 21 supply solid fuel to the unit and this fuel falls on the upper surface of the grate 18 which is moving from the front to the rear of the furnace. In addition, flyash and cindersare re-introduced into the furnace proper by the re-injection apparatus 44. Air from the fan 45 is preheated in the air heater 38 and passes through the duct 46 where its flow (an indication of load) is indicated to the main control 58 by means of the pressure-measuring devices 62 and 64 whose signals are transmitted to the control through the lines 64 and 65. Air from the duct 46 enters the air plenum chamber 48 and passes vertically upwardly into the air zones 66, 67, 68, and 69, the amount of air entering an individual zone being dependent on the setting of its dampers 77. Air flows through the upper run of the grate 18 and through the fuel to produce combustion. The products of combustion flow upwardly through the furnace transmitting heat by radiation to the Water-wall tubes covering the front wall 14, the rear wall 15, and the side walls 16. Heat is also transmitted by radiation to the radiant superheater 54, the amount so transmitted depending on factors such as the distance of the patricles of gas from the superheater and the quantity of gas. The products of combustion then pass around the forward edge of the bafe 32 through the screen 31 and into the horizontal pass between the baffle 32 and the roof 17. In that position, the gases transmit heat by convection to the convection superheater 52. The gases leave the main combustion chamber through the gas off-take 30 and flow downwardly through the forward back pass 34 in convection heat exchange with the superheater 51. The gases then reverse direction at the bottom edge of the baffle 33 and flow upwardly through the rear back pass 35 in heat exchange relationship with the downcomer tubes 25. The gas then passes over the economizer, through the dust collector, through the air heater, and through the induced-draft fan 39 to the breaching 41 from which they are directed into the atmosphere by the stack.

The feed water entering the unit first passes through the economizer 36 where it is preheated. It then passes into the steam-and-water drum 23 and flows downwardly through the downcomers 25 into the lower drum 24. From there it passes through downcomers 26 to the headers 27, 28, and 29. These headers supply the water to the water-wall tubes covering the walls of the furnace and the water flows upwardly in the usual manner, being converted to steam before reaching the steam-and-water drum 23. The steam in the steam-and-water drum flows first into the convection superheater 51 and then passes to the convection superheater 52. From there it passes through the pipe 53 to the radiant superheater 54 and finally it enters the superheated steam header 55 on its way to the turbine. VIf the temperature of the steam passing through the header 55 is higher than a pre-determined value, its temperature may be reduced by the introduction of a water spray into the pipe 53 by means of a desuperheater 56, this being under the control of impulses from the thermocouple 59 reaching the main control 58 through the line 61. The main controller sends a signal to the desuperheater through the line 57. The temperature of the steam `leaving the unit is indicated to the main control 58 by a signal transmitted by the temperature-measuring device 59 through the line 61. The main control 58, therefore, has signals indicative not only of the temperature of the steam which the unit is generating but also of the load at which the unit is operating. In a general way, it attempts to set the firing of the fuel in a certain manner depending on the load and, then, to smooth out variations of the superheated steam from a pre-determined value by use of the desuperheater 56.

In FIG. 5, which shows the operation at low load, it can be seen that the dampers 77 below the upper run of the grate 18 are set in such a manner that the air zone chambers 66 and 67 have their dampers closed, the chamber 68 has its damper one-half open, while the chamber 69 has its damper 77 entirely open. This is accomplished by the main control 58 operating through its output control lines to regulate the settings of the pistons of the hydraulic cylinders 71, 72, 73, and 74. The cylinders, of course, operate the actuating rods 78 of each chamber to set the dampers 77 in the desired manner. Now, when chamber 69 is wide-open and chamber 68 is one-half open, the flame 83 takes the form shown in FIG. 5 and flows upwardly through the combustion chamber close to the wall 14. This means that it is very close to the radiant superheater 54 so that the amount of heat radiated to the radiant superheater 54 is very great in comparison to what occurs at other positions of the flame in the furnace. At the same time, the distance from the grate 18 to the point Where the gases leave the combustion chamber is the shortest one available, so that the ternperature of the gas leaving the combustion chamber and passing over the convection superheaters 52 and 51 is very great. These factors overcome the normal tendency of the superheated steam to be low at low load and raises it to a value which is greater than the pre-determined temperature and from which it must, in some cases, be reduced by the desuperheater 56. The expression flame is used in the sense of the line of action of the greatest mass ow of products of combustion, whether they be luminous or not.

In FIG. 6, which shows the operation of the unit at high load, the dampers 77 are set so that the air zone chamber 66 has its dampers in half-open position, chamber 67 has its dampers set in wide-open position, and chambers 68 and 69 have their dampers set at closed position. As is evident in FIG. 6, this produces a flame 83 which extends up the rear Wall 1S of the furnace. The llame then curls around the under-side of the bai-lie 32 and the gas passes around the forwardmost edge of the bale before entering the upper pass and passing over the convection superheaters 52 and 51. Under these conditions the mass flow of the hot products of combustion is the furthest distance possible away from the radiant superheater 54 so that relatively little heat passes to this superheater by radiation. At the same time, the path of the products of combustion through the combustion chamber is the longest that is possible, which means that this gas has a relatively great opportunity for the transmission of heat from the gas to the water-wall tubes. Therefore, the temperature of the gases passing over the convection superheater 52 and the convection superheater 51 is at a relatively low value, which at high load is a desirable situation. 4 It will be understood, of course, that the positions of the flame 83 shown in FIGS. 5 and 6 are extreme and that control of the temperature may take place in either of two well-known ways. First of all, the step method may be used; that is to say, at certain loads, such as 60% load, the boiler may be switched from one extreme condition to the other. Above 60% load, the condition as shown in FIG. 6 may be used, and, below 60% load, the condition shown in FIG. 5 may be used. This method has the advantage that the control apparatus is relatively simple and inexpensive, but it does have the disadvantage that at many loads the temperature of the steam produced is greater than the pre-determined value and must be reduced by the desuperheater 36 which has a tendency to lower the efficiency of the unit. On the other hand, according to the continuous control method, the main control 58 may regulate the cylinders 71, 72, 73, and 74 to cause the dampers to move continuously from the condition shown in FIG. 6 to the condition shown in FIG. 5 as the load goes from one value to another. That is to say, the flame 83 may operate at various positions from adjacent the front wall 14 to adjacent the rear wall 15 at various loads and thereby produce a superheated steam which is very close to the pre-determined valve at every load in a broad range, so that very little reduction of temperature is necessary by means of the desuperheater 56.

It is obvious that minor changes may be made in the form and construction of the invention without departing from the material spirit thereof. It is not, however, desired to confine the invention to the exact form herein shown and described, but it is desired to include all such as properly come within the scope claimed.

The invention having been thus described, what is claimed as new and desired to secure by Letters Patent is:

l. A steam generating unit, comprising (a) two pairs of opposed walls consisting of water tubes defining a combustion chamber,

(b) a gas oitake at the upper portion of the chamber,

(c) a vapor heating heat exchanger located so that a majority of its heat exchange surface is closer to one wall of a pair of opposed walls than to the other wall of the said pair,

(d) a gate extending horizontally across the lower portion of the combustion chamber, the underside of the grate being provided with transversely-extending chambers which communicate with the grate, each chamber supplying air to different portions along the length of the grate,

(e) means delivering air to the chambers underlying the grate, and

(f) control means regulating the delivery of air to the chambers to cause the line of greatest mass flow of products of combustion to occupy positions relative to the said one pair of opposed walls such that thte temperature of the vapor is maintained at a constant predetermined value throughout a substantial range of loads.

2. A steam generating unit, comprising (a) two pairs of opposed walls consisting of water tubes defining a combustion chamber,

(b) a gas off-take at the upper portion of one wall of one pair of opposed walls,

(c) a vapor heating heat exchanger located so that a majority of its heat exchange surface is closer to thel 1other wall of the said one pair than to the said one wa (d) a gate extending horizontally across the lower end of the combustion chamber, the underside of the grate being provided with transversely-extending chambers which communicate with the grate, each chamber supplying air to different portions along the length of the grate,

(e) means delivering air to the chambers underlying the grate, and

(f) control means regulating the delivery of air to the chambers to cause the line of greatest mass flow of products of combustion to occupy positions relative to the said one pair of opposed walls such that the temperature of the vapor is maintained at a constant predetermined value throughout a substantial range of loads.

3. A steam generating unit, comprising (a) front, rear, side and roof walls consisting of water tubes dening a vertically-elongated combustion chamber,

(b) a gas oE-take at the upper end of the rear wall,

(c) a radiant superheater located adjacent the upper end of the front wall,

(d) a bale underlying the gas olf-take and extending forwardly from the rear wall,

(e) a traveling grate extending horizontally across the l-ower end of the combustion chamber, the underside of the grate being provided with transversely-extending chambers which communicate with the grate, each chamber supplying air to different portions along the length of the grate,

(f) means delivering air to the chambers underlying the grate, and

(g) control means regulating the delivery of air to the chambers to cause the line of greatest mass flow of products of combustion to occupy positions relative to the front and rear walls such that the temperature of superheated steam is maintained at a constant predetermined value throughout a substantial range of loads.

4. A steam generating unit as recited in claim 3, wherein spreader stokers are mounted on the walls to project solid fuel onto the grate in a pre-determined pattern.

5. A steam generating unit as recited in claim 3, wherein the said chambers extend from side wall to side wall in spaced, parallel relationship with one another.

6. A steam generating unit as recited in claim 3, wherein the means for delivering air includes an air plenum chamber which underlies the entire grate, a horizontal dividing wall extending horizontally .at a position between the plenum and the grate, vertical dividing walls extending from side wall to side wall and from the said dividing Wall to the grate, the vertical dividing walls defining 4the chambers, apertures through the horizontal dividing wall into the respective chambers, and a damper associated with each aperture to control the ow of air therethrough.

'7. A steam generating unit, comprising (a) two pairs of opposed walls consisting of water tubes dening a combustion chamber,

(b) a gas oif-take at the upper portion of the chamber,

(c) a vapor heating heat exchanger located so that a majority of its heat exchange surface is closer to one wall of a pair of opposed walls than to the other wall of the said pair,

(d) a grate extending horizontally across the lower portion of the combustion chamber, the underside of the grate being provided with transversely-extending chambers which communicate with the grate, each chamber supplying air to diiferent portions along the length of the grate.

(e) means delivering air to the chambers underlying the grate, and

(f) control means regulating the delivery of air to the chambers to cause the line of greatest mass flow of products of combustion to occupy positions relative to the said one pair of opposed walls such that the temperature of the Vapor is maintained at a constant pre-determined value throughout a substantial range of loads, the means for delivering air including an air plenum chamber which underlies the Ventire grate, a horizontal dividing wall extending `horizontally at a positionbetween the plenum and the grate,

vertical dividing walls extending from side wall to side wall and from the said dividing wall to the grate, the vertical dividing walls defining the chambers, apertures through the horizontal dividing wall into the respective chambers, and a damper associated with each aperture to control the flow of air therethrough.

8. A steam generating unit, comprising (a) two pairs of opposed walls consisting of water tubes defining a combustion chamber,

(b) a gas off-take at the upper portion of the chamber,

(c) a heat exchanger containing uid to be heated 1ocated so that a majority of its heat exchange surface is closer to one wall of a pair of opposed walls than to the other wall of the said pair,

(d) a grate extending horizontally across the lower portion of the combustion chamber, the underside of the grate being provided with transversely-extending chambers which communicate with the grate, each chamber supplying air to different portions along the length of the grate,

(e) means delivering air to the chambers underlying the grate, and

(f) control means regulating the delivery of air to the chambers to cause the line of greatest mass flow of products of combustion to occupy positions relative to Said one pair of opposed walls such that the temperature of the Huid in the heat exchanger is maintained at a constant pre-determined value throughout a substantial range of loads.

References Cited by the Examiner UNITED STATES PATENTS 3/37 Bloomsbury et al. 110-40 8/60 Miller 122-481 

1. A STEAM GENERATING UNIT, COMPRISING (A) TWO PAIRS OF OPPOSED WALLS CONSISTING OF WATER TUBES DEFINING A COMBUSTION CHAMBER, (B) A GAS OFF-TAKE AT THE UPPER PORTION OF THE CHAMBER, (C) A VAPOR HEATING HEAT EXCHANGER LOCATED SO THAT A MAJORITY OF ITS HEAT EXCHANGE SURFACE IS CLOSER TO ONE WALL OF A PAIR OF OPPOSED WALLS THAN TO THE OTHER WALL OF THE SAID PAIR, (D) A GATE EXTENDING HORIZONTALLY ACROSS THE LOWER PORTION OF THE COMBUSTION CHAMBER, THE UNDERSIDE OF THE GRATE BEING PROVIDED WITH TRANSVERSELY-EXTENDING CHAMBERS WHICH COMMUNICATE WITH THE GRATE, EACH CHAMBER SUPPLYING AIR TO DIFFERENT PORTIONS ALONG THE LENGTH OF THE GRATE, (E) MEANS DELIVERING AIR TO THE CHAMBERS UNDERLYING THE GRATE, AND (F) CONTROL MEANS REGULATING THE DELIVERY OF AIR TO THE CHAMBERS TO CAUSE THE LINE OF GREATEST MASS FLOW OF PRODUCTS OF COMBUSTION TO OCCUPY POSITIONS RELATIVE TO THE SAID ONE PAIR OF OPPOSED WALLS SUCH THAT THE TEMPERATURE OF THE VAPOR IS MAINTAINED AT A CONSTANT PREDETERMINED VALUE THROUGHOUT A SUBSTANTIAL RANGE OF LOADS. 